A microprocessor is introduced in a system (device) which automatically administers a fast-acting drug at an appropriate rate so that a desired effect of the drug is achieved. The specific system (device) which will be designed uses an antihypertensive drug, nitroprusside, to lower the blood pressure to a desired level. The output: the blood pressure is measured automatically and only intermittently using any indirect method. Thus, the value of the output variable to be controlled is available only at the sampling instants. This value will be fed into a microprocessor, which determines on the basis of the received information the value of the controller gains. Thus, the operation of the system requires only an appropriate use of an inflatable cuff and a needle for iv infusion. The microprocessor will be so programmed that it can be operated by push button. Additional features to alarm medical personnel in case of emergency will be incorporated into the system. The design of the feedback loop involving a microprocessor will consist of programming two algorithms into the microprocessor: one algorithm is for the identification of the mathematical model for the process to be controlled, and the other is for computing and adjusting the gains of the feedback controller. The system (device) which will automatically administer the drug will be tested on dogs using nitroprusside as the hypertensive drug. Since the automated blood pressure measuring device which works well on humans has not yet been found successful for dogs its function will be simulated using a continuous-time pressure transducer in conjunction with a sampling and holding device. The basic approach used here can be extended in future work to antiarrhythmic drugs, anesthetics etc. These systems will require more data processing on-line than is needed in the blood pressure requlating system. In fact, the use of a microprocessor in those systems is almost a necessity if such signals as ECG and EEG will be used for the purpose of controlling the system variables. It can be expected that in the future microprocessors will be essential components of many biomedical instruments.